1. Technical Field
The present invention relates to an electrical micro-relay device and more specifically to a liquid metal micro-relay device.
2. Background Art
There are many different types of electrical micro-relay devices, and one popular type is the reed micro-relay, which is a small, mechanical contact type of electrical micro-relay device. A reed micro-relay has two reeds made of a magnetic alloy sealed in an inert gas inside a glass vessel surrounded by an electromagnetic driver coil. When current is not flowing in the coil, the tips of the reeds are biased to break contact and the device is switched off. When current is flowing in the coil, the tips of the reeds attract each other to make contact and the device is switched on.
The reed micro-relay has problems related to large size and relatively short service life. As to the first problem, the reeds not only require a relatively large volume, but also do not perform well during high frequency switching due to their size and electromagnetic response. As to the second problem, the flexing of the reeds due to biasing and attraction causes mechanical fatigue, which can lead to breakage of the reeds after extended use.
In the past, the reeds were tipped with contacts composed of rhodium, tungsten, or were plated with rhodium or gold for conductivity and electrical arcing resistance when making and breaking contact between the reeds. However, these contacts would fail over time. This problem with the contacts has been improved with one type of reed micro-relay called a xe2x80x9cwetxe2x80x9d relay. In a wet relay, a liquid metal, such as mercury, is used to make the contact. This solves the problem of contact failure, but the problem of mechanical fatigue of the reeds remained unsolved.
In an effort to solve these problems, electrical micro-relay devices have been proposed that make use of the liquid metal in a channel between two micro-relay electrodes without the use of reeds. In the liquid metal devices, the liquid metal acts as the contact connecting the two micro-relay electrodes when the device is switched ON. The liquid metal is separated between the two micro-relay electrodes by a fluid non-conductor when the device is switched OFF. The fluid non-conductor is generally high purity nitrogen or other such inert gas.
With regard to the size problem, the liquid metal devices afford a reduction in the size of an electrical micro-relay device since reeds are not required. Also, the use of the liquid metal affords longer service life and higher reliability.
The liquid metal devices are generally manufactured by joining together two substrates with a heater in between to heat the gas. The gas expands to separate the liquid metal to open and close a circuit. Previously, the heaters were inline resistors patterned on one of the substrates between the two substrates. The substrates were of materials such as glass, quartz, and gallium arsenide upon which the heater material was deposited and etched. Since only isotropic etching could be used, the heater element would consist of surface wiring. The major drawback of surface wiring is that such wiring has poor high frequency characteristics, high connection resistance, and poor thermal transfer to the gas.
More recently, suspended heaters have been developed. A suspended heater refers to a configuration in which the heating elements are positioned so that they can be surrounded all the way around by the gas.
Generally, the suspended heaters are made by placing a heater material in a patterned shape on a sacrificial layer. The sacrificial layer is then etched away from under the heater material so that the heater material is suspended in space. The advantages of suspended heaters are that the gas heating efficiency is high and almost all of the heat that is generated by the heater is used to heat the gas because the surface area of the heater face that contacts the gas is large and the support areas are small. As a result, the transfer of heat to the support structure is minimized.
The preferred method for manufacturing a suspended heater is to place the heater material on a silicon substrate and then to etch the silicon substrate by anisotropic etching to undercut the heater material.
The problem with using silicon through out a micro-relay is that it is difficult to form multiple layer substrates with multiple layers of wiring.
On the other hand, ceramic materials can be formed to provide multiple layers of wiring and surface wiring does not have to be used. Contact electrodes can be formed with connecting vias. This permits a low connection resistance and favorable high frequency characteristics. Unfortunately, the formation of a suspended heater on a ceramic substrate is problematic and so the heater element must be formed on the surface of the ceramic substrate. With the heater formed on the surface of the ceramic substrate, a considerable portion of the heat generated by the heater is transferred directly to the substrate so that the gas heating efficiency decreases substantially. As a result, it is difficult to obtain rapid switching at low power.
Solutions to these problems have been long sought, but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.
The present invention provides a micro-relay device including a fluid non-conductor. A first substrate and a second substrate are bonded together. A channel is defined in at least one of the substrates, and has a liquid metal in the channel. Electrodes are spaced along the channel and selectively interconnectable by the liquid metal. An open via is defined in one of the substrates and contains the fluid non-conductor. A heater substrate includes a suspended heater element in fluid communication with the open via. The suspended heater element is operable to cause the fluid nonconductor to separate the liquid metal. The micro-relay device provides rapid switching at low power in a small package.
Certain embodiments of the invention have other advantages in addition to or in place of those mentioned above. The advantages will become apparent to those skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings.